Heparin-induced thrombocytopenia (HIT) may be the most common drug-induced immune disease, and can result in devastating or fatal arterial/venous thrombosis and thromboembolism. Patient IgG antibodies that bind to platelet factor 4 (PF4) in a complex with heparin to form lgG/PF4/heparin immune complexes are central to the pathogenesis of HIT. These immune complexes, in turn, bind FcyRlla on the platelet surface and induce platelet activation, leading to thrombocytopenia and contributing to thrombosis. HIT exhibits both features of a T ceW-independent immune response, characterized by the rapid onset and decline of antibodies and no immunological memory, as well as aspects of a secondary T ceW-dependent immune response, characterized by the prevalence of the IgG class of anti-PF4/heparin antibodies and a requirement for T cells. These atypical immunological characteristics of HIT have confounded our understanding ofthe mechanism by which B cells contribute to the immune pathogenesis of the disease. It is known that signals from the B cell receptor (BCR) are required for B cell function. A critical event in BCR signaling is activation of phospholipase Cy2 (PLCy2), which hydrolyzes membrane lipids to generate diacylglycerol (DAG) and inositol 1,4,5- trisphosphate (IP3). DAG leads to activation of a signaling pathway that involves protein kinase C (PKC), a B-cell lymphoma 10 (BcllO)-containing complex, and the protein kinase TAK1. Our previous targeted gene-disruption studies have shown that the PLCy2/Bcl10/TAK1 pathway is essential for BCR-mediated antibody production. Moreover, a mouse model for production of anti-mouse PF4/heparin HIT antibodies has been established, as well as hFcyRlla transgenic mice that can recapitulate HIT. These models make it possible to characterize molecular aspects of the immune response to PF4/heparin complexes that are essential to human HIT development. We propose to use these mouse models, in combination with several other genetically-modified mouse strains, to elucidate how B cells produce the PF4/heparin-specific antibodies that cause HIT, and determine whether manipulation of the PLCy2/Bcl10/TAK1 pathway to control B cell antibody production can prevent HIT. Specifically, we will determine 1) the contribution of marginal zone, B1 and memory B cells to HIT antibody production, 2) whether breakdown of B cell anergy contributes to activation of PF4/heparin-specific B cells, and 3) whether inhibition of the PLCy2/Bcl10/TAK1 pathway can prevent HIT. Identifying the B cell subsets responsible for HIT antibody production, determining how PF4 /heparin-specific B cells are activated, and defining the role of the BCR signaling pathway in HIT antibody production will provide new clues to the pathogenesis of HIT and help to develop novel prevention methods. RELEVANCE (See instructions): This study aims to understand the cause of heparin-induced thrombocytopenia, one of the most common drug-induced life-threatening blood diseases, and is expected to identify specific targets for developing novel and effective ways for the control and prevention of this disease.